Subband coder or decoder band-limiting the overlap region between a processed subband and an adjacent non-processed one

ABSTRACT

If all original subbands are not selected for processing in conventional subband coders or decoders aliasing distortion is generated by the characteristics of their subband band-splitting filters or subband band synthesis filters. To improve sound quality in a subband decoder the decoded frequency components in the overlap region adjacent to a subband selected not to be decoded are band-limited prior to synthesis. Alternatively, in a subband coder the sound quality in a processed subband adjacent to one not to be coded is improved by band-limiting the filtering frequency overlap region between these subbands prior to coding. By thus decoding only the non-overlapping part of the subband adjacent to an omitted subband signal distortion is reduced.

TECHNICAL FIELD

This invention relates to an information decoding method and device, aninformation coding method and device, and a providing medium. Itparticularly relates to an information decoding method and device, aninformation coding method and device, and a providing medium forrestraining output of an unpleasant sound by erasing an aliasingcomponent with respect to a code string formed by coding only a signalof a partial frequency band of acoustic waveform signals.

BACKGROUND ART

Conventionally, there are various methods and devices for highefficiency coding of audio or sound signals. For example, such methodsand devices can be exemplified by a transform coding system which isadapted for forming frames of signals in the time domain, thenconverting (spectral conversion) each frame of signals in the timedomain to signals in the frequency domain, splitting the signals into aplurality of frequency bands and coding each band of signals, and aso-called subband coding (SBC) system which is adapted for splittingaudio signals in the time domain into a plurality of frequency bands andcoding each band of signals, without forming frames of audio signals.Also, there is considered a method and device for high efficiency codingusing the above-described subband coding in combination with transformcoding. In this case, after band splitting is carried out by the subbandcoding system, each band of signals are spectrally converted to signalsin the frequency domain, and coding is carried out on each spectrallyconverted band.

As a band splitting filter used in the above-described subband codingsystem, there is employed, for example, a polyphase quadrature filter(PQF), which is described in Joseph H. Rothweiler, “Polyphase QuadratureFilters—A new subband coding technique,” ICASSP 83, BOSTON. This PQF cansplit a signal into a plurality of bands of equal widths at a time andis characterized in that so-called aliasing is not generated insynthesizing the split bands later.

As the above-described spectral conversion, there is employed spectralconversion for forming frames of input audio signals of predeterminedduration and carrying out a discrete Fourier transform (DFT), discretecosine transform (DCT) or modified discrete cosine transform (MDCT) oneach frame so as to convert the time domain to the frequency domain,MDCT is described in J. P. Princen and A. B. Bradley, “Subband/TransformCoding Using Filter Bank Designs Based on Time Domain AliasingCancellation,” Univ. of Surrey Royal Melbourne Inst. of Tech. ICASSP1987.

By thus using the filter and spectral conversion to quantize the signalssplit for each band, the band in which quantization noise is generatedcan be controlled and coding can be carried out at an auditorily higherefficiency using characteristics such as a so-called masking effect.Also, by normalizing the maximum value of absolute values of signalcomponents for each band before carrying out quantization, coding can becarried out at a higher efficiency.

As a frequency splitting width in quantizing each frequency component(hereinafter referred to as a spectral component) split into frequencybands determined by human auditory characteristics is often employed.Specifically, critical bands whose bandwidths increase as the frequencybecomes higher are used to split audio signals into a plurality of bands(for example, 25 bands). In coding each band of data at this point,coding is carried out by using predetermined bit distribution for eachband or adaptive bit allocation for each band. For example, in codingcoefficient data obtained by MDCT processing by using the foregoing bitallocation, coding is carried out by using an adaptive number ofallocated bits with respect to each band of MDCT coefficient dataobtained by MDCT processing on each frame.

The following two methods are known as the bit allocation method.

For example, in R. Zelinski and P. Noll, “Adaptive Transform Coding ofSpeech Signals,” IEEE Transactions of Acoustics, Speech, and SignalProcessing, Vol.ASSP-25, No.4, August 1977, bit allocation is carriedout on the basis of the magnitude of signals of each band. In thismethod, the quantization noise spectrum becomes flat and the noiseenergy is minimized. However, since the auditory masking effect is notused, the actual auditory perception of noise is not optimum.

On the other hand, for example, in M. A. Kransner, “The critical bandcoder—digital encoding of the perceptual requirements of the auditorysystem,” MIT, ICASSP 1980, there is described a method for obtaining asignal-to-noise ratio required for each band by utilizing auditorymasking so as to carry out fixed bit allocation. However, in thismethod, since bit allocation is fixed, a satisfactory characteristicvalue cannot be obtained even in the case where characteristics aremeasured by using a sine wave input.

To solve these problems, a high efficiency coding device is proposed inwhich all the bits that can be used for bit allocation are divided intobits for a fixed allocation pattern predetermined for each band or eachblock obtained by subdividing each band and bits for bit distributiondependent on the magnitude of signal frequency components in eachsubband, and in which the division ratio is caused to depend on signalsrelated to input signals to that the division ratio for the fixed bitallocation pattern is increased as the spectral distribution of thesignals becomes smoother.

According to this method, in the case where the energy is concentratedon a specified spectral component as in the case of a sine wave input,the overall signal-to-noise characteristic can be significantly improvedby allocating a greater number of bits to a block including thatspectral component. In general, the human auditory sense is extremelyacute with respect to signals having steep spectral distribution.Therefore, improvement of the signal-to-noise characteristic by usingsuch method is effective not only for improving the numerical value ofmeasurement but also for improving the auditorily perceived soundquality.

In addition to the foregoing method, various other methods are proposedas the bit allocation method. If a model related to the auditory senseis made fine to improve the capability of the information coding device,coding can be carried out at an auditorily higher efficiency.

In the case where the above-described DFT or DCT is used as the methodfor spectral conversion of waveform signals consisting of waveformelements (sample data) such as digital audio signals of the time domain,blocks are formed by every M units of sample data and spectralconversion of DFT or DCT is carried out on each block. By carrying outspectral conversion on such blocks, M units of independent real numberdata (DFT coefficient data or DCT coefficient data) are obtained. The Munits of real number data thus obtained are quantized and coded, thusgenerating coded data.

In decoding the coded data to reproduce regenerative waveform signals,the coded data are decoded and inversely quantized, and inverse spectralconversion by inverse DFT or inverse DCT is carried out on each block ofthe resultant real number data corresponding to the block at the time ofcoding, thus generating waveform element signals. Then, blocksconsisting of the waveform element signals are connected to reproducewaveform signals.

In the regenerative waveform signals thus obtained, a connectiondistortion in connecting the blocks remains, which is less desirable interms of the auditory sense. Thus, in order to reduce the connectiondistortion between the blocks, in carrying out spectral conversion usingDFT or DCT in actual coding, M1 units of sample data each of theadjacent blocks are caused to overlap each other for spectralconversion.

However, in the case where M1 units of sample data each of the adjacentblocks are caused to overlap each other for spectral conversion, M unitsof real number data are obtained with respect to (M−M1) units of sampledata on the average, and the number of real number data obtained byspectral conversion becomes greater than the number of original sampledata actually used for spectral conversion. Since these real number dataare subsequently quantized and coded, the increase in the number of realnumber data obtained by spectral conversion with respect to the originalsample data is less desirable in terms of coding efficiency.

On the contrary, in the case where the above-described MDCT is usedsimilarly as the method for spectral conversion of waveform signalsconsisting of sample data such as digital audio signals, in order toreduce the connection distortion between the blocks, spectral conversionis carried out by using 2M units of sample data obtained by causing Munits of sample data each of the adjacent blocks to overlap each other,and M units of independent real number data (MDCT coefficient data) areobtained. Thus, in this spectral conversion using MDCT, M units of realnumber data are obtained with respect to M units of sample data on theaverage, and coding can be carried out at a higher efficiency than inthe above-described case of spectral conversion using DFT or DCT.

In decoding the coded data obtained by quantizing and coding the realnumber data obtained by the spectral conversion using MDCT so as togenerate regenerative waveform signals, the coded data are decoded andinversely quantized, and inverse spectral conversion by inverse MDCT iscarried out on the resultant real number data to obtain waveformelements in the block. Then, the waveform elements in the block areadded while being caused to interfere with each other, thusreconstituting waveform signals.

FIG. 1 is a block diagram showing an exemplary structure of aconventional information coding device for coding acoustic waveformsignals. Waveform signals inputted from an input terminal are splitinto, for example, four bands by a band splitting filter 121 employingthe above-described polyphase quadrature filter. The signals of the fourbands split by the band splitting filter 121 are sent to correspondingspectral conversion circuits 122-1 to 122-4, respectively. The signalsof the respective bands inputted to the spectral conversion circuits122-1 to 122-4 are converted to corresponding signal frequencycomponents, and are then supplied to a quantization precisiondetermination circuit 123 and a normalization/quantization circuit 124.The normalization/quantization circuit 124 carries out normalization andquantization by using quantization precision information found by thequantization precision determination circuit 123.

The normalization/quantization circuit 124 supplies normalizedcoefficient information consisting of normalized coefficients at thetime normalization and coded signal frequency components to a codestring generation circuit 125. The code string generation circuit 125generates a code string from the quantization precision informationinputted from the quantization precision determination circuit 123 andthe normalized coefficient information and the coded signal frequencycomponents inputted from the normalization/quantization circuit 124, andoutputs the generated code string.

FIG. 2 is a block diagram showing a specific exemplary structure of aninformation decoding device for decoding the code string generated bythe information coding device of FIG. 1 so as to generate and outputacoustic signals.

A code string resolution circuit 131 extracts, from an inputted codestring (code string generated by the information coding device of FIG.1), information and components corresponding to the normalizedcoefficient information and the signal frequency components outputtedfrom the normalization/quantization circuit 124 of FIG. 1 andinformation corresponding to the quantization precision informationoutputted from the quantization precision determination circuit 123, andoutputs the extracted information and components to a signal componentdecoding circuit 132.

The signal component decoding circuit 132 restores, from the informationand components, the respective signal frequency components outputtedfrom the spectral conversion circuits 122-q to 122-4 of FIG. 1, andsupplies the signal frequency components to corresponding inversespectral conversion circuits 133-1 to 133-4, respectively. The inversespectral conversion circuits 133-1 to 133-4 carry out inverse spectralconversion processing corresponding to the spectral conversion circuits122-1 to 122-4, respectively, and supply the resultant band signals to aband synthesis filter 134 corresponding to the band splitting filter 121of FIG. 1. As the band synthesis filter 134, for example, an inversepolyphase quadrature filter (IPQF) is used. The band synthesis filter134 generates acoustic waveform signals from the signals of four bandssupplied from the inverse spectral conversion circuits 133-1 to 133-4,and outputs the acoustic waveform signals.

A method for coding in the information coding device of FIG. 1 will nowbe described with reference to FIG. 3.

Spectral signal components ES shown in FIG. 3 are obtained by convertinginput acoustic waveform signals to a total of 64 spectral signalcomponents ES for each predetermined time frame, by the spectralconversion circuits 122-1 to 122-4 of FIG. 1. These 64 spectral signalcomponents ES are gathered in groups (referred to as coding units) byfive predetermined bands (bands b1 to b5), and are then normalized andquantized by the normalization/quantization circuit 124. In this case,the bandwidth of the coding units is set to be narrower on the lowerfrequency side and to be broader on the higher frequency side so thatgeneration of quantization noise can be controlled in accordance withauditory characteristics. In FIG. 3, the levels of absolute values ofspectral signals (frequency components) obtained by MDCT processing areconverted to dB values, and the normalized coefficient values of therespective coding units are also shown.

In the information coding device of FIG. 1 for coding information inthis manner, the scale of the coding device can be reduced by using, forexample, only the spectral conversion circuit 122-1 for coding only adesired band without using the other spectral conversion circuits 122-2to 122-4. Also, if all the information regions used for coding all thebands are used in coding a desired band, the sound quality of thedesired band can be improved.

FIG. 4 is a block diagram showing a specific exemplary structure of aninformation coding device having its hardware scale reduced by carryingout spectral conversion of only the lowest frequency band. In this case,spectral conversion is carried out only on the lowest frequency band.However, as a matter of course, it is possible to carry out spectralconversion only on another arbitrary frequency band.

In FIG. 4, the same circuit components as those of the informationcoding device of FIG. 1 are denoted by the same numerals and thereforewill not be described further in detail. In this information codingdevice, only the signal frequency component of the lowest frequencyband, of the four bands split by the band splitting filter 121, is sentto the spectral conversion circuit 122-1. The other signal frequencycomponents are not used because spectral conversion is not carried outon these signal frequency components. The spectral conversion circuit122-1 spectrally converts the inputted signal of the predeterminedlowest band to a signal frequency component, and supplies the signalfrequency component to the quantization precision determination circuit123 and the normalization/quantization circuit 124. Thenormalization/quantization circuit 124 carries out normalization andquantization by using quantization precision information found by thequantization precision determination circuit 123.

Although FIG. 4 shows the example in which only the single spectralconversion circuit 122-1 is used, an information coding device using twoor three spectral conversion circuits can be similarly realized.

FIG. 5 shows an example of a code string generated by the informationcoding device of FIGS. 1 or 4. The coding unit information U1 to U5 ofthis code string is constituted by quantization precision information,normalized coefficient information, and normalized and quantized signalcomponent information SC1 to SC8. The code string is recorded onto arecording medium such as a magneto-optical disc or transmitted through atransmission medium such as a network.

In the coding unit information U1, the quantization precision of thecorresponding coding unit is two bits, indicating that eight spectralsignal components are included in this coding unit. If the quantizationprecision information is 0 (zero) as in the coding unit information U4,it is indicated that coding is not actually carried out in this codingunit.

In the information decoding device of FIG. 2 for decoding a code string,the hardware scale required for the information decoding device can bereduced by using, for example, only the inverse spectral conversioncircuit 133-1 for outputting only the signal frequency componentsincluding a desired band without using the other inverse spectralconversion circuits 133-2 to 133-4.

FIG. 6 is a block diagram showing a specific exemplary structure of aninformation decoding device having its hardware scale reduced bycarrying out inverse spectral conversion of only the lowest frequencyband. In this case, inverse spectral conversion is carried out only onthe lowest frequency band. However, as a matter of course, it ispossible to carry out inverse spectral conversion only on anotherarbitrary frequency band.

In FIG. 6, the same portions as those of the information decoding deviceof FIG. 2 are denoted by the same numerals and therefore will not bedescribed further in detail. Of the signal frequency components decodedby the signal component decoding circuit 132, only the signal frequencycomponent of the lowest frequency band is sent to the inverse spectralconversion circuit 133-1. The other signal frequency components are notused because the inverse spectral conversion is not carried out on thesesignal frequency components. The inverse spectral conversion circuit133-1 carries out inverse spectral conversion of the signal frequencycomponent of the predetermined lowest band, and supplies the resultantband signal to the band synthesis filter 134. The band synthesis filter134 generates output acoustic signals from the band signal from theinverse spectral conversion circuit 133-1 and band signals having avalue 0 inputted from terminals 101 to 103, and outputs the outputacoustic signals.

In the information decoding device of such structure, by selecting thereproducing band, it suffices to use only the single inverse spectralconversion circuit 133-1 as shown in FIG. 6, while the four inversespectral conversion circuits 133-1 to 133-4 are required in the decodingdevice of FIG. 2. Thus, the hardware scale of the decoding device can bediminished and the cost can also be reduced.

Although FIG. 6 shows the example in which only the single inversespectral conversion circuit is used, an information decoding deviceusing two or three inverse spectral conversion circuits can be similarlyrealized.

Meanwhile, in the case where the code string generated by theinformation coding device of FIG. 4 is decoded by using the informationdecoding device of FIG. 2, or in the case where the code stringgenerated by the information coding device of FIG. 1 is decoded by usingthe information decoding device of FIG. 6, the aliasing componentgenerated by the characteristics of the band splitting filter 121 or theband synthesis filter 134 is not cancelled but is included in the outputacoustic signals, thus raising a problem of deterioration in soundquality.

This problem will now be described using specific examples. FIG. 7 showsfrequency characteristics in the case where the above-describedpolyphase quadrature filter of quadrisection is used as the bandsplitting filter 121. The lateral axis represents the frequency, and 6kHz, 12 kHz and 18 kHz scaled on the lateral axis represent the splitfrequencies in the case where the sampling frequency is 48 kHz.Overlapping of the characteristics of the filter is generated in afrequency region of a predetermined width around the split frequency asthe center, and the signals of that region are included in the output ofthe filter, as the aliasing components, which are frequency signalssymmetrical with respect to the split frequency, with the magnitude ofamplitude corresponding to the cut-off characteristics of the filter. Inthe vicinity of the split frequency of 6 kHz, a region from 5 kHz to 7kHz is a region where the filter characteristics overlap.

FIG. 8 shows the state of an aliasing component generated in the casewhere signal components exist in the vicinity of the split frequency of6 kHz. An aliasing component B, corresponding to an original signal Awhich is a frequency signal component exceeding 6 kHz in theabove-described region where the filter characteristics overlap, appearsin the frequency band not higher than 6 kHz.

In general, this aliasing component B is cancelled by the originalsignal A in decoding. Similarly, an aliasing component appears in thefrequency band exceeding 6 kHz, but this aliasing component is cancelledby the original signal in decoding.

However, in the case where the code string generated by the informationcoding device of FIG. 4 is decoded by the information decoding device ofFIG. 2, the frequency component not lower than 6 kHz does not exist, andtherefore the aliasing component not higher than 6 kHz cannot becancelled in the band synthesis filter 134. Also, the signal forcancelling the aliasing component by the original signal not higher than6 kHz appears as a signal component not lower than 6 kHz.

In the information decoding device of FIG. 6, since inverse spectralconversion processing using the inverse spectral conversion circuit isnot carried out with respect to the frequency not lower than 6 kHz, theoriginal signal not lower than 6 kHz does not exist and the aliasingcomponent not higher than 6 kHz is not cancelled in the band synthesisfilter 134. Also, the signal for cancelling the aliasing component bythe original signal not higher than 6 kHz appears as a signal componentnot lower than 6 kHz.

The signal thus generated appears in the output acoustic signals,depending on the frequency signal component of the original signal.Therefore, the demodulated acoustics signals are heard as unpleasantsounds.

DISCLOSURE OF THE INVENTION

In view of the foregoing status of the art, it is an object of thepresent invention to enable restraint of deterioration in sound qualityat the time of coding and decoding only a part of frequency bands ofwaveform signals split into a plurality of frequency bands.

An information decoding method according to the present invention isadapted for decoding a signal comprising at least one frequency bandfrom a code string obtained by converting and coding a signal split intoa plurality of adjacent frequency bands, including a first frequencyband adjacent to a second frequency band. The method includes: selectingthe first frequency band as a frequency band to be decoded and selectingthe second frequency band as a frequency band not to be decoded; in thefirst frequency band, band-limiting a signal component in a filteringfrequency overlap region between the first frequency band and the secondfrequency band; and inversely converting the band-limited signalcomponent in the first frequency band.

An information decoding device according to the present invention isadapted for decoding a signal comprising at least one frequency bandfrom a code string obtained by converting and coding a signal split intoa plurality of adjacent frequency bands, including a first frequencyband adjacent to a second frequency band. The device includes: means forselecting the first frequency band as a frequency band to be decoded andselecting the second frequency band as a frequency band not to bedecoded; means for band-limiting, in the first frequency band, a signalcomponent in a frequency filtering overlap region between the firstfrequency band and the second frequency band; and means for inverselyconverting the band-limited signal component in the first frequencyband.

In the information decoding method and the information decoding device,when decoding a signal of at least one frequency band from a code stringobtained by converting and coding a signal split into a plurality offrequency bands, the value of a signal component of a band adjacent to aband not be decoded, of the bands to be decoded, is limited.

A providing medium according to the present invention is adapted forproviding processing for decoding a signal comprising at least onefrequency band from a code string obtained by converting and coding asignal split into a plurality of adjacent frequency bands, including afirst frequency band adjacent to a second frequency band. The processingincludes: selecting the first frequency band as a frequency band to bedecoded and selecting the second frequency band as a frequency band notto be decoded; in the first frequency band, band-limiting a signalcomponent in a filtering frequency overlap region between the firstfrequency band and the second frequency band; and inversely convertingthe band-limited signal component in the first frequency band.

An information decoding method according to the present invention isadapted for decoding a signal comprising at least one frequency bandfrom a code string obtained by converting and coding a signal split intoa plurality of adjacent frequency bands, including a first frequencyband adjacent to a second frequency band. The method includes:identifying that the first frequency band is encoded and the secondfrequency band is not encoded; in the first frequency band,band-limiting a signal component in a filtering frequency overlap regionbetween the first frequency band and the second frequency band; andinversely converting the band-limited signal component in the firstfrequency band.

An information decoding device according to the present invention isadapted for decoding a signal comprising at least one frequency bandfrom a code string obtained by converting and coding a signal split intoa plurality of adjacent frequency bands, including a first frequencyband adjacent to a second frequency band. The device includes: means foridentifying that the first frequency band is encoded and the secondfrequency band is not encoded; means for band-limiting, in the firstfrequency band, a signal component in a filtering frequency overlapregion between the first frequency band and the second frequency band;and means for inversely converting the band-limited signal component inthe first frequency band.

A providing medium according to the present invention is adapted forproviding processing for decoding a signal comprising at least onefrequency band from a code string obtained by converting and coding asignal split into a plurality of adjacent frequency bands, including afirst frequency band adjacent to a second frequency band. The processingincludes: identifying that the first frequency band is encoded and thesecond frequency band is not encoded; in the first frequency band,band-limiting a signal component in a filtering frequency overlap regionbetween the first frequency band and the second frequency band; andinversely converting the band-limited signal component in the firstfrequency band.

An information decoding method according to the present invention isadapted for decoding a code string obtained by converting and coding atleast one band of a signal split into a plurality of frequency bands,including a first frequency band adjacent to a second frequency band.The method includes: selecting the first frequency band as a frequencyband to be decoded and selecting the second frequency band as afrequency band not to be decoded; band-limiting a signal component ofthe first frequency band in a filtering frequency overlap region betweenthe first frequency band and the second frequency band; and inverselyconverting the band-limited signal component of the first frequencyband.

An information decoding device according to the present invention isadapted for decoding a code string obtained by converting and coding atleast one band of a signal split into a plurality of frequency bands,including a first frequency band adjacent to a second frequency band.The device includes: means for selecting the first frequency band as afrequency band to be decoded and selecting the second frequency band asa frequency band not to be decoded; means for band-limiting a signalcomponent of the first frequency band in a filtering frequency overlapregion between the first frequency band and the second frequency band;and means for inversely converting the band-limited signal component ofthe first frequency band.

A providing medium according to the present invention is adapted forproviding processing for decoding a code string obtained by convertingand coding at least one band of a signal split into a plurality offrequency bands, including a first frequency band adjacent to a secondfrequency band. The processing includes: selecting the first frequencyband as a frequency band to be decoded and selecting the secondfrequency band as a frequency band not to be decoded; band-limiting asignal component of the first frequency band in a filtering frequencyoverlap region between the first frequency band and the second frequencyband; and inversely converting the band-limited signal of the firstfrequency band.

An information decoding method according to the present invention isadapted for decoding a code string obtained by coding at least onefrequency band of a signal split into a plurality of frequency bands,including a first frequency band adjacent to a second frequency band.The method includes: in the first frequency band, restoring frequencysignal components from the code string; and inversely converting therestored frequency signal components; where the inverse frequencyconversion step includes limiting the value of a signal componentexisting in a frequency overlap region between the first frequency bandand the second frequency band.

An information decoding device according to the present invention isadapted for decoding a code string obtained by coding at least onefrequency band of a signal split into a plurality of adjacent frequencybands, including a first frequency band adjacent to a second frequencyband. The device includes: means for selecting the first frequency bandas a frequency band to be decoded and selecting the second frequencyband as a frequency band not to be decoded; means for restoring, in thefirst frequency band, frequency signal components from the code string;and means for inversely converting the restored frequency signalcomponents; where the inverse frequency conversion means includes meansfor limiting the value of a signal component existing in a frequencyoverlap region between the first frequency band and the second frequencyband.

A providing medium according to the present invention is adapted forproviding processing for decoding a code string obtained by coding atleast one frequency band of a signal split into a plurality of adjacentfrequency bands, including a first frequency band adjacent to a secondfrequency band. The processing includes: selecting the first frequencyband as a frequency band to be decoded and selecting the secondfrequency band as a frequency band not to be decoded; in the firstfrequency band, restoring frequency signal components from the codestring; and inversely converting the restored frequency signalcomponents; where the inverse frequency conversion step includeslimiting the value of a signal component existing in a frequency overlapregion between the first frequency band and the second frequency band.

An information coding method according to the present invention isadapted for coding an input signal. The method includes: splitting theinput signal into a plurality of frequency bands, including a firstfrequency band adjacent to a second frequency band; selecting the firstfrequency band to be coded; selecting the second frequency band not tobe coded; converting the first frequency band to a signal frequencycomponent; band-limiting the signal component of the first frequencyband in a filtering frequency overlap region between the first frequencyband and the second frequency band.

An information coding device according to the present invention isadapted for coding an input signal. The device includes: means forsplitting the input signal into a plurality of frequency bands,including a first frequency band adjacent to a second frequency band;means for selecting the first frequency band to be coded; means forselecting the second frequency band not to be coded; means forconverting the first frequency band to a signal frequency component;means for band-limiting the signal component of the first frequency bandin a filtering frequency overlap region between the first frequency bandand the second frequency band.

A providing medium according to the present invention is adapted forproviding processing for coding an input signal. The processingincludes: splitting the input signal into a plurality of frequencybands, including a first frequency band adjacent to a second frequencyband; selecting the first frequency band to be coded; selecting thesecond frequency band not to be coded; converting the first frequencyband to a signal frequency component; band-limiting the signal componentof the first frequency band in a filtering frequency overlap regionbetween the first frequency band and the second frequency band.

An information coding method according to the present invention isadapted for coding an input signal. The method includes: splitting theinput signal into a plurality of frequency bands, including a firstfrequency band adjacent to a second frequency band; selecting the firstfrequency band to be coded; selecting the second frequency band not tobe coded; converting a time signal of the first frequency to frequencysignal components and coding the frequency signal components; andgenerating a code string from the coded signal; where converting thetime signal includes band-limiting a signal component of the firstfrequency band in a filtering frequency overlap region between the firstfrequency band and the second frequency band.

An information coding device according to the present invention isadapted for coding an input signal. The device includes: means forsplitting the input signal into a plurality of frequency bands,including a first frequency band adjacent to a second frequency band;selecting the first frequency band to be coded; selecting the secondfrequency band not to be coded; means for converting a time signal ofthe first frequency to frequency signal components and coding thefrequency signal components; and means for generating a code string fromthe coded signal; where the converting means includes means forband-limiting a signal component of the first frequency band in afiltering frequency overlap region between the first frequency band andthe second frequency band.

A providing medium according to the present invention is adapted forproviding processing for coding an input signal. The processingincludes: splitting the input signal into a plurality of frequencybands, including a first frequency band adjacent to a second frequencyband; selecting the first frequency band to be coded; selecting thesecond frequency band not to be coded; converting a time signal of thefirst frequency to frequency signal components and coding the frequencysignal components; and generating a code string from the coded signal;where converting the time signal includes band-limiting a signalcomponent of the first frequency band in a filtering frequency overlapregion between the first frequency band and the second frequency band.

A providing medium according to the present invention is adapted forproviding a signal coded by an information coding method for coding aninput signal. The information coding method includes: splitting theinput signal into a plurality of frequency bands, including a firstfrequency band adjacent to a second frequency band; selecting the firstfrequency band to be coded; selecting the second frequency band not tobe coded; band-limiting a signal component of the first frequency bandin a filtering frequency overlap region between the first frequency bandand the second frequency band; and converting the band-limited signal.

A providing medium according to the present invention is adapted forproviding a signal coded by an information coding method for coding aninput signal. The information coding method includes: splitting theinput signal into a plurality of frequency bands, including a firstfrequency band adjacent to a second frequency band; selecting the firstfrequency band to be coded; selecting the second frequency band not tobe coded; converting a time signal of the first frequency to frequencysignal components and coding the frequency signal components; andgenerating a code string from the coded signal; wherein converting thetime signal includes band-limiting a signal component of the firstfrequency band in a filtering frequency overlap region between the firstfrequency band and the second frequency band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an exemplary structure of aconventional information coding device.

FIG. 2 is a block diagram showing an exemplary structure of aconventional information decoding device.

FIG. 3 shows an example of each coding unit in a frame.

FIG. 4 is a block diagram showing another exemplary structure of theconventional information coding device.

FIG. 5 shows a code string coded by the conventional information codingdevice.

FIG. 6 is a block diagram showing another exemplary structure of theconventional information decoding device.

FIG. 7 shows characteristics of a band splitting filter 121 of FIG. 1.

FIG. 8 illustrates an aliasing component of a signal.

FIG. 9 is a block diagram showing an exemplary structure of anembodiment of an information decoding device according to the presentinvention.

FIGS. 10A to 10C illustrate a processing method in the informationdecoding device of FIG. 9.

FIG. 11 is a block diagram showing an exemplary structure of anotherembodiment of the information decoding device according to the presentinvention.

FIG. 12 is a block diagram showing an exemplary structure of stillanother embodiment of the information decoding device according to thepresent invention.

FIG. 13 is a block diagram showing an exemplary structure of stillanother embodiment of the information decoding device according to thepresent invention.

FIG. 14 is a block diagram showing an exemplary structure of stillanother embodiment of the information decoding device according to thepresent invention.

FIG. 15 illustrates a code string which is decoded by the informationdecoding device according to the present invention.

FIG. 16 is a block diagram showing the structure of still anotherembodiment of the information decoding device according to the presentinvention.

FIG. 17 is a flowchart for explaining information decoding processing ofthe information decoding device of FIG. 16.

FIGS. 18A and 18B illustrate processing of a band limitation circuit204-1 of FIG. 16.

FIG. 19 is a block diagram showing the structure of still anotherembodiment of the information decoding device according to the presentinvention.

FIG. 20 is a block diagram showing the structure of an embodiment of aninformation coding device according to the present invention.

FIG. 21 is a flowchart for explaining information coding processing ofthe information coding device of FIG. 20.

FIGS. 22A and 22B illustrate processing of a band limitation circuit 303of FIG. 20.

FIG. 23 is a block diagram showing the structure of another embodimentof the information coding device according to the present invention.

FIG. 24 is a block diagram showing the structure of still anotherembodiment of the information coding device according to the presentinvention.

FIG. 25 is a block diagram showing the structure of still anotherembodiment of the information coding device according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be describedwith reference to the drawings.

FIG. 9 is a block diagram showing an exemplary structure of anembodiment of an information decoding device according to the presentinvention. The information decoding device shown in FIG. 9 has such astructure that a band limitation circuit 41 for limiting the band isnewly provided between a signal component decoding circuit 33 and aninverse spectral conversion circuit 34 in the conventional informationdecoding device shown in FIG. 6. The other parts of the structure aresimilar to those of the information decoding device of FIG. 6 andtherefore will not be described further in detail.

The operation of the device will be described next. When a code stringcorresponding to the code string outputted from the coding device ofFIG. 1 is supplied to an input terminal 31, this code string is sent toa code string resolution circuit 32. The code string resolution circuit32 extracts, from the code string, information and componentscorresponding to the normalized coefficient information and the signalfrequency components supplied from the normalization/quantizationcircuit 124 of FIG. 1 and information corresponding to the quantizationprecision information supplied from the quantization precisiondetermination circuit 123 of FIG. 1, and sends the extracted informationand components to the signal component decoding circuit 33.

The signal component decoding circuit 33 restores original signalfrequency components (signals outputted from the spectral conversioncircuits 122-1 to 122-4 of FIG. 1) from these information (normalizedcoefficient information and quantization precision information) andsignal frequency components, and then sends only the signal frequencycomponent of the lowest frequency (signal frequency componentcorresponding to the signal outputted from the spectral conversioncircuit 122-1 of FIG. 1) to the band limitation circuit 41. The othersignal frequency components are not used here because inverse spectralconversion is not carried out on these signal frequency components.

In this example, the signal frequency components are not used afterbeing decoded. However, it is also possible that these signal frequencycomponents are not decoded by the signal component decoding circuit 33,thus omitting unnecessary decoding processing.

The band limitation circuit 41 sets, at 0, the value of the signalfrequency component of a region where characteristics of the bandsplitting filter 12 overlap, from among the signal frequency componentssupplied from the signal component decoding circuit 33, and generates aband-limited signal frequency component. The band limitation circuit 41supplies the band-limited signal frequency component to the inversespectral conversion circuit 34.

The inverse spectral conversion circuit 34 carries out inverse spectralconversion of the signal frequency component supplied from the bandlimitation circuit 41, and supplies the resultant band signal to a bandsynthesis filter 38.

The band synthesis filter 38 generates acoustic waveform signals fromthe band signal inputted from the inverse spectral conversion circuit 34and the band signals of the value 0 inputted from terminals 101 to 103,and outputs the acoustic waveform signals from an output terminal 39.

FIGS. 10A to 10C show examples of signal frequency components in theembodiment shown in FIG. 9. The lateral axis represents the frequencyand the longitudinal axis represents the absolute value of the signalfrequency component. FIG. 10A shows an example of signal frequencycomponents of all bands, that is, all the signal frequency components asthe output of the signal component decoding circuit 33 of FIG. 9. FIG.10B shows only the signal frequency components of the lowest band, ofthe signal frequency components of all bands of FIG. 10A. These signalfrequency components are inputted to the band limitation circuit 41.FIG. 10C shows an output signal of the band limitation circuit 41 at thetime when the signal frequency components of FIG. 10B are inputted tothe band limitation circuit 41. It can be understood from FIG. 10C thatthe value of three signal frequency components corresponding the signalfrequency components in the region where the characteristics of the bandsplitting filter 121 of FIG. 1 overlap (in this case, a region from 5kHz to 7 kHz) is 0.

FIG. 11 is a block diagram showing an exemplary structure of anotherembodiment of the information decoding device according to the presentinvention. The information decoding device shown in FIG. 11 has such astructure that an inverse spectral conversion circuit 35 is provided inthe information decoding device of FIG. 9 so that inverse spectralconversion is carried out on two bands from the lowest frequency band.Also, in place of the band limitation circuit 41 of FIG. 9, a bandlimitation circuit 51 is provided between a signal component decodingcircuit 33 and the inverse spectral conversion circuit 35.

The other parts of the structure and the operation thereof are similarto those of FIG. 9 and therefore will not be described further indetail. In the information decoding device of FIG. 11, a signalfrequency component of the second band from the lowest frequency band isinputted to the band limitation circuit 51, and the band-limited signalfrequency component is supplied to the inverse spectral conversioncircuit 35. The inverse spectral conversion circuit 35 supplies the bandsignal on which inverse spectral conversion has been carried out, to aband synthesis filter 38. The signal frequency component of the lowestfrequency band is supplied to an inverse spectral conversion circuit 34,where inverse spectral conversion processing is carried out. After that,the band signal is supplied to the band synthesis filter 38.

In the embodiment shown in FIG. 11, output acoustic signals of bandstwice that of the embodiment of FIG. 9 can be obtained, and two inversespectral conversion circuits can be omitted.

FIG. 12 is a block diagram showing an exemplary structure of stillanother embodiment of the information decoding device according to thepresent invention. The information decoding device shown in FIG. 12 hassuch a structure that inverse spectral conversion is carried out onthree bands from the lowest frequency band. That is, the informationdecoding device of FIG. 12 has such a structure that an inverse spectralconversion circuit 36 is provided in the information decoding device ofFIG. 11 so that inverse spectral conversion is carried out on threebands from the lowest frequency band. Also, in place of the bandlimitation circuit 51 of FIG. 11, a band limitation circuit 61 isprovided between a signal component decoding circuit 33 and the inversespectral conversion circuit 36.

The other parts of the structure and the operation thereof are similarto those of FIG. 11 and therefore will not be described further indetail. A signal frequency component of the third band from the lowestfrequency band is inputted to the band limitation circuit 61, and anoutput of the band limitation circuit 61 is supplied to the inversespectral conversion circuit 36, where inverse spectral conversionprocessing is carried out. After that, the resultant band signal issupplied to a band synthesis filter 38. In the embodiment shown in FIG.12, output acoustic signals of bands three times that of the embodimentof FIG. 9 can be obtained, and one inverse spectral conversion circuitcan be omitted.

FIG. 13 is a block diagram showing an exemplary structure of stillanother embodiment of the information decoding device according to thepresent invention. The information decoding device shown in FIG. 13 hassuch a structure that a band limitation circuit 71 is newly providedafter a band synthesis filter 38 in the conventional informationdecoding device of FIG. 6. The other parts of the structure and theoperation thereof are similar to those of FIG. 6 and therefore will notbe described further in detail.

In the information decoding device shown in FIG. 13, acoustic signals asan output of the band synthesis filter 38 are inputted to the bandlimitation circuit 71. The band limitation circuit 71 limits the band ofthe output signals so as not to include the region where the bandsplitting filter characteristics overlap, of the band which is to beprocessed by inverse spectral conversion adjacent to the band which isnot to be processed by inverse spectral conversion. That is, bandlimitation on the time base is carried out. Thus, sounds of unnecessarysignal components can be prevented from being outputted.

FIG. 14 is a block diagram showing an exemplary structure of stillanother embodiment of the information decoding device according to thepresent invention. The information decoding device shown in FIG. 14 hassuch a structure that the band synthesis filter 38 is removed from theinformation decoding device of FIG. 9 while a D/A converter 81 isprovided therein. The other parts of the structure and the operationthereof are similar to those described with reference to FIG. 9.

The D/A converter 81 converts a digital signal from an inverse spectralconversion circuit 34 to an analog signal, and outputs the analog signalfrom an output terminal 39. In this example, since the frequency of thesignal from the inverse spectral conversion circuit 34 is reduced ¼, theD/A converter 81 increases the frequency to four times and then outputsthe signal. With such a structure, effects similar to those of theforegoing embodiment can be obtained.

As described above, in the above-described embodiments, before inversespectral conversion is carried out on the signal frequency component ofthe highest frequency band of the bands which are to be processed byinverse spectral conversion, the aliasing component generated when theinverse spectral conversion circuit is omitted is erased by using thefirst method (method in the embodiments of FIGS. 9, 11 and 12) forlimiting the band by setting, at 0, the value of the signal frequencycomponent of the region where the band splitting filter characteristicsoverlap, or the second method (method in the embodiment of FIG. 13) forproviding the band limitation filter at the output of the band synthesisfilter. Thus, generation of unpleasant sounds is restrained.

The first method for limiting the band before inverse spectralconversion and the second method for limiting the band after bandsynthesis by the band synthesis filter are now compared with each other.In the first method, since it suffices to set, at 0, the value of thesignal frequency component of the region where the characteristics ofthe band splitting filter 121 overlap, the processing quantity isnegligibly small and steep band limitation characteristics can beobtained. On the other hand, in the second method, since band limitationis carried out on the time base, a filter of a high order must be usedto obtain steep band limitation characteristics and a further temporaldelay due to filter processing is generated.

Thus, though it is desired to use the first method for the purpose ofreducing the scale of the information decoding device, similar effectscan also be obtained by the second method.

In the above-described embodiments, the number of split bands generatedby the band splitting filter is four. However, the number of split bandsis not limited to four and a greater or smaller number may also be used.

In the above-described embodiments, one or plural bands including thelowest frequency band are decoded. However, it is also possible todecode only an intermediate band. In such case, band limitation must becarried out on the upper and lower ends of that band.

The information decoding device of the present invention can be appliedto, for example, DVD (digital versatile disc) or satellite broadcastusing the MPEG (Moving Picture Experts Group) 2 or AAC (Advanced AudioCoding) system.

With the information decoding method and the information decoding deviceas described above, in extracting a plurality of bands of signals fromthe code string and decoding only signals of a predetermined band of theplural bands, the band adjacent to the band not to be decoded, of thebands to be decoded, is limited. Therefore, the aliasing component canbe erased to restrain generation of unpleasant sounds, thus outputtingsatisfactory sounds.

Before explaining another exemplary structure of the informationdecoding device of the present invention, a code string to be decoded bythe information decoding device will be described with reference to FIG.15. This code string is obtained by coding signals of a part of thefrequency bands of input signals and setting the value of signals of theother frequency regions at 0, similarly to the code string generated bythe information coding device of FIGS. 1 or 4. This code string isconstituted by a header and coding unit information U1 to U5. The headerincludes maximum band information, which indicates the maximum valuefrom among numbers sequentially allocated to the respective splitfrequency bands (that is, the number of coded bands). The coding unitinformation U1 to U5 is similar to that of the code string shown in FIG.5 and therefore will not be described further in detail.

The structure of another embodiment of the information decoding deviceaccording to the present invention will now be described with referenceto FIG. 16. A code string resolution circuit 201 extracts normalizedcoefficient information, quantization precision information and signalfrequency components from an inputted code string, and supplies theextracted information and signal frequency components to a signalcomponent decoding circuit 202. Also, the code string resolution circuit201 extracts maximum band information from the inputted code string andoutputs the extracted maximum band information to switches 203-1 to203-3.

The signal component decoding circuit 202 restores signal frequencycomponents of the four frequency bands from the inputted signalfrequency components on the basis of the normalized coefficientinformation and the quantization precision information. Then, the signalcomponent decoding circuit 202 outputs the signal frequency component ofthe lowest frequency band to the switch 203-1, and outputs the signalfrequency component of the second lowest frequency band to the switch203-2. The signal component decoding circuit 202 outputs the signalfrequency component of the third lowest frequency band to the switch203-3, and outputs the signal frequency component of the highestfrequency band to an inverse spectral conversion circuit 205-4.

The switches 203-1 to 203-3 switch the output destinations of theinputted signal frequency components to corresponding band limitationcircuits 204-1 to 204-3 or inverse spectral conversion circuits 205-1 to205-3, or are set in the off-state, on the basis of the maximum bandinformation. For example, if the maximum band information indicates thelowest frequency band, the switch 203-1 switches the output destinationof the signal frequency component to the band limitation circuit 204-1,and the switches 203-2 and 203-3 are set in the off-state. If themaximum band information indicates the second lowest frequency band, theswitch 203-2 switches the output destination of the signal frequencycomponent to the band limitation circuit 204-2, and the switch 203-1switches the output destination of the signal frequency component to theinverse spectral conversion circuit 205-1. The switch 203-3 is set inthe off-state. If the maximum band information indicates the thirdlowest frequency band, the switch 203-3 switches the output destinationof the signal frequency component to the band limitation circuit 204-3,and the switches 203-1 and 203-2 switch the output destinations of thesignal frequency components to the inverse spectral conversion circuits205-1 and 205-2, respectively. If the maximum band information indicatesthe highest frequency band, the switches 203-1 to 203-3 switch theoutput destinations of the signal frequency components to the inversespectral conversion circuits 205-1 to 205-3, respectively.

The band limitation circuits 204-1 to 204-3 limit the signals (i.e., setthe value of the signals to 0 or a value proximate to 0) of the regionwhere the filter characteristics of the band splitting filter 121(FIG. 1) overlap, from the inputted signal frequency components, andoutput the band-limited signal frequency components to the correspondinginverse spectral conversion circuits 205-1 to 205-3.

The inverse spectral conversion circuits 205-1 to 205-4 carry outinverse spectral conversion processing corresponding to the spectralconversion circuits 122-1 to 122-4 (FIG. 1), and output the resultantwaveform element signals to a band synthesis filter 206 corresponding tothe band splitting filter 121. The band synthesis filter 206 synthesizesacoustic waveform signals from the signals of four bands supplied fromthe inverse spectral conversion circuits 205-1 to 205-4, and outputs theacoustic waveform signals.

The operation of this information decoding device will now be describedwith reference to the flowchart of FIG. 17. At step S21, the code stringresolution circuit 201 extracts normalized coefficient information,quantization precision information and signal frequency components froman inputted code string, and supplies the extracted information andsignal frequency components to the signal component decoding circuit202. At the same time, the code string resolution circuit 201 extractsmaximum band information from the inputted code string, and outputs theextracted maximum band information to the switches 203-1 to 203-3.

At step S22, the signal component decoding circuit 202 restores signalfrequency components of four frequency bands from the inputted signalfrequency components on the basis of the normalized coefficientinformation and the quantization precision information. The signalcomponent decoding circuit 202 outputs the signal frequency component ofthe lowest frequency band to the switch 203-1, and outputs the signalfrequency component of the second lowest frequency band to the switch203-2. The signal component decoding circuit 202 outputs the signalfrequency component of the third lowest frequency band to the switch203-3, and outputs the signal frequency component of the highestfrequency band to the inverse spectral conversion circuit 205-4.

At step S23, the switches 203-1 to 203-3 switch the output destinationsof the inputted signal frequency components to the corresponding bandlimitation circuits 204-1 to 204-3 or inverse spectral conversioncircuits 205-1 to 205-3, or are set in the off-state, on the basis ofthe maximum band information.

At step S24, the band limitation circuits to which the signal frequencycomponent is inputted at step S23, from among the band limitationcircuits 204-1 to 204-3, limit the signals (i.e., set the value of thesignals to 0 or a value proximate to 0) of the region where the filtercharacteristics of the band splitting filter 121 overlap, from thesignal frequency components, and output the band-limited signalfrequency components to the corresponding inverse spectral conversioncircuits 205-1 to 205-3. For example, if the maximum band informationindicates the lowest frequency band and signal frequency components asshown in FIG. 18A are inputted to the band limitation circuit 204-1, theband limitation circuit 204-1 limits to 0 the value of the signalfrequency components existing in the band (5 kHz to 7 kHz) in thevicinity of the split frequency of 6 kHz where the filtercharacteristics overlap, as shown in FIG. 18B. In FIGS. 18A and 18B, thelateral axis represents the frequency and the longitudinal axisrepresents the level of signal frequency.

At step S25, the inverse spectral conversion circuits 205-1 to 205-4carry out inverse spectral conversion processing corresponding to thespectral conversion circuits 122-1 to 122-4 of the information codingdevice, and output the resultant band signals to the band synthesisfilter 206 corresponding to the band splitting filter 121 of theinformation coding device.

At step S26, the band synthesis filter 206 synthesizes acoustic waveformsignals from the signals (waveform element signals) of four bandssupplied from the inverse spectral conversion circuits 205-1 to 205-4,and outputs the acoustic waveform signals.

However, at step S23, if the inputted maximum band information indicatesthe highest frequency band, the switches 203-1 to 203-3 switch theoutput destinations of the signal frequency components to thecorresponding inverse spectral conversion circuits 205-1 to 205-3.Therefore, processing of step S24 is not carried out.

FIG. 19 is a block diagram showing an exemplary structure of stillanother embodiment of the information decoding device according to thepresent invention. This information decoding device has such a structurethat the switches 203-1 to 203-3 and the band limitation circuits 204-1to 204-3 are omitted from the information decoding device of FIG. 16 andsuch that a switch 211 and a band limitation circuit 212 are providedafter a band synthesis filter 206. The switch 211 switches the outputdestination in accordance with maximum band information inputted from acode string resolution circuit 201. The other parts of the structure andthe operation thereof are similar to those of the information decodingdevice of FIG. 16 and therefore will not be described further in detail.

The information decoding device shown in FIG. 16 and the informationdecoding device shown in FIG. 19 are now compared with each other. Inthe information decoding device of FIG. 16, since it suffices that theband limitation circuits 204-1 to 204-3 set, at 0, the value of thesignal frequency components of the region where the characteristics ofthe band splitting filter 121 of the information coding device overlap,the processing quantity is negligibly small and steep band limitationcharacteristics can be obtained. On the other hand, in the informationdecoding device of FIG. 19, since the band limitation circuit 212 cariesout band limitation on the time base, a filter of a high order must beused to obtain steep band limitation characteristics and a furthertemporal delay due to filter processing is generated.

As described above, by setting at 0 the value of the signal frequencycomponents of the region where the band splitting filter characteristicsoverlap so as to carry out band limitation, generation of the aliasingcomponent and hence generation of unpleasant sounds can be restrained.

In the present embodiments, one or plural bands including the lowestfrequency band are decoded. However, it is also possible to decode onlyan intermediate band. In such case, band limitation must be carried outon the upper and lower ends of that band.

The information decoding device of the present invention can be appliedto, for example, DVD (digital versatile disc) or satellite broadcastusing the MPEG (Moving Picture Experts Group) 2 or AAC (Advanced AudioCoding) system.

A computer program for carrying out the above-described processing canbe provided to the user through a network providing medium such as theInternet or a digital satellite as well as a providing medium includingan information recording medium such as a magnetic disk or a CD-ROM.

Thus, according to the information decoding device, the informationdecoding method and the providing medium, since signal componentsexisting in a predetermined frequency region are limited on the basis ofsplit information extracted from a code string, generation of thealiasing component and hence generation of unpleasant sounds can berestrained.

The structure of an information coding device according to the presentinvention will now be described with reference to FIG. 20. A bandsplitting filter 301 of this information coding device splits inputtedwaveform signals into four frequency bands, and outputs the signal ofthe lowest frequency band to a spectral conversion circuit 302-1. Inthis information coding device, since only the lowest frequency band iscoded, the signals of the other frequency bands are not processed. Asthe band splitting filter 301, a polyphase quadrature filter or the likeis used.

The spectral conversion circuit 302-1 converts the inputted signal tosignal frequency components and outputs the signal frequency componentsto a band limitation circuit 303. The band limitation circuit 303removes the signal frequency component existing in the band where thefilter characteristics of the band splitting filter 301 overlap, andsupplies the resultant signal frequency components to a quantizationprecision determination circuit 304 and a normalization/quantizationcircuit 305. The quantization precision determination circuit 304determines the quantization precision in accordance with the quantity ofinputted signal frequency components, and outputs the determinedinformation to the normalization/quantization circuit 305 and a codestring generation circuit 306. The normalization/quantization circuit305 carries out normalization and quantization of the inputted signalfrequency components on the basis of the quantization precisioninformation, and outputs the resultant signal frequency componentstogether with normalized coefficient information indicating a normalizedcoefficient to the code string generation circuit 306.

The code string generation circuit 306 generates a code string from thequantization precision information inputted from the quantizationprecision determination circuit 304 and the normalized coefficientinformation and coded signal frequency components inputted from thenormalization/quantization circuit 305, and outputs the code string.

The operation of this information coding device will now be describedwith reference to the flowchart of FIG. 21. At step S31, the bandsplitting filter 301 splits inputted waveform signals into fourfrequency bands as shown in FIG. 7, and outputs the signal of the lowestfrequency band to the spectral conversion circuit 302-1.

At step S32, the spectral conversion circuit 302-1 converts the inputtedsignal not higher than 6 kHz to signal frequency components as shown inFIG. 22A, and outputs the signal frequency components to the bandlimitation circuit 303.

At step S33, the band limitation circuit 303 limits (i.e., sets thelevel at 0 or a value proximate to 0) the signal frequency componentexisting in the region (5 kHz to 7 kHz) where the filter characteristicsof the band splitting filter 301 overlap, as shown in FIG. 22B, andsupplies the resultant signal frequency components to the quantizationprecision determination circuit 304 and the normalization/quantizationcircuit 305.

At step S34, the quantization precision determination circuit 304determines the quantization precision in accordance with the quantity ofinputted signal frequency components, and outputs the determinedinformation to the normalization/quantization circuit 305 and the codestring generation circuit 306. The normalization/quantization circuit305 carries out normalization and quantization of the inputted signalfrequency components on the basis of the quantization precisioninformation, and outputs the resultant signal frequency componentstogether with normalized coefficient information indicating a normalizedcoefficient to the code string generation circuit 306.

At step S35, the code string generation circuit 306 generates a codestring from the quantization precision information inputted from thequantization precision determination circuit 304 and the normalizedcoefficient information and coded signal frequency components inputtedfrom the normalization/quantization circuit 305, and outputs the codestring.

FIG. 23 shows another exemplary structure of the information codingdevice according to the present invention. This information codingdevice is adapted for coding signals of the lowest frequency band up tothe second lowest frequency band from among frequency bands split by aband splitting filter 301, and has such a structure that a spectralconversion circuit 302-2 for spectrally converting the signal of thesecond lowest frequency band is added to the information coding deviceof FIG. 20. A band limitation circuit 303 limits signal frequencycomponents existing in the band where the filter characteristicsoverlap, in the vicinity of the split frequency (12 kHz) shown in FIG.7. The other parts of the structure are similar to those of theinformation coding device of FIG. 20 and therefore will not be describedfurther in detail.

FIG. 24 shows still another exemplary structure of the informationcoding device according to the present invention. This informationcoding device is adapted for coding signals of the lowest frequency bandup to the third lowest frequency band from among frequency bands splitby a band splitting filter 301, and has such a structure that a spectralconversion circuit 302-3 for spectrally converting the signal of thethird lowest frequency band is added to the information coding device ofFIG. 23. A band limitation circuit 303 limits signal frequencycomponents existing in the band where the filter characteristicsoverlap, in the vicinity of the split frequency (18 kHz) shown in FIG.7. The other parts of the structure are similar to those of theinformation coding device of FIG. 23 and therefore will not be describedfurther in detail.

FIG. 25 shows still another exemplary structure of the informationcoding device according to the present invention. This informationcoding device has such a structure that the band limitation circuit 303of the information coding device of FIG. 20 is omitted and that a bandlimitation circuit 311 is provided prior to a band splitting filter 301.The band limitation circuit 311 limits signal frequency componentsexisting in the region where the filter characteristics of the bandsplitting filter 301 overlap, from inputted waveform signals. The otherparts of the structure are similar to those of the information codingdevice of FIG. 20 and therefore will not be described further in detail.

As described above, in the case where a code string generated by codinga part of split frequency bands is decoded by using the informationdecoding device of FIG. 2, since no signal frequency component exists inthe band (5 kHz to 7 kHz) where the filter characteristics overlaparound the split frequency (6 kHz) of this code string, no aliasingcomponent is generated. Therefore, deterioration in sound quality due tonoise is restrained.

Although the number of split bands generated by the band splittingfilter 301 of the information coding device is set to four, the numberof split bands is not limited to four and may be greater or smaller thanfour.

In the present embodiments, one or plural bands including the lowestfrequency band are decoded. However, it is also possible to decode onlyan intermediate band. In such case, band limitation must be carried outon the upper and lower ends of that band.

The information coding device of the present invention can be appliedto, for example, DVD (digital versatile disc) or satellite broadcastusing the MPEG (Moving Picture Experts Group) 2 or AAC (Advanced AudioCoding) system.

A computer program for carrying out the above-described processing canbe provided to the user through a network providing medium such as theInternet or a digital satellite as well as a providing medium includingan information recording medium such as a magnetic disk or a CD-ROM.

Thus, according to the information coding device, the information codingmethod and the providing medium, since coding is carried out by limitingsignal components existing in a predetermined frequency region,generation of the aliasing component and hence generation of unpleasantsounds can be restrained when the signal is decoded.

What is claimed is:
 1. An information decoding method for decoding asignal comprising at least one frequency band from a code stringobtained by converting and coding a signal split into a plurality ofadjacent frequency bands, including a first frequency band adjacent to asecond frequency band, the method comprising: selecting the firstfrequency band as a frequency band to be decoded and selecting thesecond frequency band as a frequency band not to be decoded; in thefirst frequency band, band-limiting a signal component in a filteringfrequency overlap region between the first frequency band and the secondfrequency band; and inversely converting the band-limited signalcomponent in the first frequency band.
 2. The information decodingmethod as claimed in claim 1, wherein the first frequency band comprisesa band on the low-frequency side of the plurality of adjacent frequencybands.
 3. The information decoding method as claimed in claim 1, furthercomprising synthesizing signals of the plurality of bands.
 4. Theinformation decoding method as claimed in claim 3, wherein the signalcomponent is band-limited before the signals of the plurality of bandsare synthesized.
 5. The information decoding method as claimed in claim3, wherein the signal component is band-limited after the signals of theplurality of bands are synthesized.
 6. The information decoding methodas claimed in claim 3, wherein the signals of the plurality of bands aresynthesized using an inverse polyphase quadrature filter.
 7. Theinformation decoding method as claimed in claim 3, wherein inverselyconverting the band-limited signal component includes converting afrequency signal component to a time signal, and wherein synthesizingsignals comprises band synthesis including the band-limited signal. 8.The information decoding method as claimed in claim 1, wherein thesignal component is band-limited before the signal of the band isinversely converted.
 9. The information decoding method as claimed inclaim 1, wherein the signal component is band-limited after the signalof the band is inversely converted.
 10. The information decoding methodas claimed in claim 1, wherein band splitting comprises polyphasequadrature filtering.
 11. The information decoding method as claimed inclaim 1, wherein inversely converting the band-limited signal componentincludes converting a frequency signal component to a time signal. 12.The information decoding method as claimed in claim 1, whereinband-limiting the signal component includes setting the overlap regionbetween the first frequency band and the second frequency band at 0 or avalue proximate to
 0. 13. The information decoding method as claimed inclaim 1, wherein the filtering frequency overlap region is a quadraturefilter overlap region.
 14. An information decoding device for decoding asignal comprising at least one frequency band from a code stringobtained by converting and coding a signal split into a plurality ofadjacent frequency bands, including a first frequency band adjacent to asecond frequency band, the device comprising: means for selecting thefirst frequency band as a frequency band to be decoded and selecting thesecond frequency band as a frequency band not to be decoded; means forband-limiting, in the first frequency band, a signal component in afiltering frequency overlap region between the first frequency band andthe second frequency band; and means for inversely converting theband-limited signal component in the first frequency band.
 15. Theinformation decoding device as claimed in claim 14, wherein the firstfrequency band comprises a band on the low-frequency side of theplurality of adjacent frequency bands.
 16. The information decodingdevice as claimed in claim 14, further comprising means for synthesizingsignals of the plurality of bands.
 17. The information decoding deviceas claimed in claim 16, wherein the signal component is band-limitedbefore the signals of the plurality of bands are synthesized.
 18. Theinformation decoding device as claimed in claim 16, wherein the signalcomponent is band-limited after the signals of the plurality of bandsare synthesized.
 19. The information decoding device as claimed in claim16, wherein the synthesis means comprises an inverse polyphasequadrature filter.
 20. The information decoding device as claimed inclaim 16, wherein the inverse conversion means converts a frequencysignal component to a time signal, and wherein the synthesis meansperforms band synthesis.
 21. The information decoding device as claimedin claim 14, wherein the signal component is band-limited before thesignal of the band is inversely converted.
 22. The information decodingdevice as claimed in claim 14, wherein the signal component isband-limited after the signal of the band is inversely converted. 23.The information decoding device as claimed in claim 14, wherein bandsplitting comprises polyphase quadrature filtering.
 24. The informationdecoding device as claimed in claim 14, wherein the inverse conversionmeans includes means for converting a frequency signal component to atime signal.
 25. The information decoding device as claimed in claim 14,wherein the limitation means sets the overlap region between the firstfrequency band and the second frequency band at 0 or a value proximateto
 0. 26. The information decoding device as claimed in claim 14,wherein the filtering frequency overlap region is a quadrature filteroverlap region.
 27. A providing medium for providing processing fordecoding a signal comprising at least one frequency band from a codestring obtained by converting and coding a signal split into a pluralityof adjacent frequency bands, including a first frequency band adjacentto a second frequency band, the processing comprising: selecting thefirst frequency band as a frequency band to be decoded and selecting thesecond frequency band as a frequency band not to be decoded; in thefirst frequency band, band-limiting a signal component in a filteringfrequency overlap region between the first frequency band and the secondfrequency band; and inversely converting the band-limited signalcomponent in the first frequency band.
 28. An information decodingmethod for decoding a signal comprising at least one frequency band froma code string obtained by converting and coding a signal split into aplurality of adjacent frequency bands, including a first frequency bandadjacent to a second frequency band, the method comprising: identifyingthat the first frequency band is encoded and the second frequency bandis not encoded; in the first frequency band, band-limiting a signalcomponent in a filtering frequency overlap region between the firstfrequency band and the second frequency band; and inversely convertingthe band-limited signal component in the first frequency band.
 29. Aninformation decoding device for decoding a signal comprising at leastone frequency band from a code string obtained by converting and codinga signal split into a plurality of adjacent frequency bands, including afirst frequency band adjacent to a second frequency band, the devicecomprising: means for identifying that the first frequency band isencoded and the second frequency band is not encoded; means forband-limiting, in the first frequency band, a signal component in afiltering frequency overlap region between the first frequency band andthe second frequency band; and means for inversely converting theband-limited signal component in the first frequency band.
 30. Aproviding medium for providing processing for decoding a signalcomprising at least one frequency band from a code string obtained byconverting and coding a signal split into a plurality of adjacentfrequency bands, including a first frequency band adjacent to a secondfrequency band, the processing comprising: identifying that the firstfrequency band is encoded and the second frequency band is not encoded;in the first frequency band, band-limiting a signal component in afiltering frequency overlap region between the first frequency band andthe second frequency band; and inversely converting the band-limitedsignal component in the first frequency band.
 31. An informationdecoding method for decoding a code string obtained by converting andcoding at least one band of a signal split into a plurality of frequencybands, including a first frequency band adjacent to a second frequencyband, the method comprising: selecting the first frequency band as afrequency band to be decoded and selecting the second frequency band asa frequency band not to be decoded; band-limiting a signal component ofthe first frequency band in a filtering frequency overlap region betweenthe first frequency band and the second frequency band; and inverselyconverting the band-limited signal component of the first frequencyband.
 32. The information decoding method as claimed in claim 31,further comprising extracting a selected number of coded bands from thecode string, wherein the signal component is band-limited based on theselected number of coded bands.
 33. The information decoding method asclaimed in claim 31, wherein inversely converting the band-limitedsignal component includes converting a frequency signal component to atime signal.
 34. The information decoding method as claimed in claim 31,wherein band-limiting the signal component includes setting the overlapregion between the first frequency band and the second frequency band at0 or a value proximate to
 0. 35. The information decoding method asclaimed in claim 31, wherein the filtering frequency overlap region is aquadrature filter overlap region.
 36. An information decoding device fordecoding a code string obtained by converting and coding at least oneband of a signal split into a plurality of frequency bands, including afirst frequency band adjacent to a second frequency band, the devicecomprising: means for selecting the first frequency band as a frequencyband to be decoded and selecting the second frequency band as afrequency band not to be decoded; means for band-limiting a signalcomponent of the first frequency band in a filtering frequency overlapregion between the first frequency band and the second frequency band;and means for inversely converting the band-limited signal component ofthe first frequency band.
 37. The information decoding device as claimedin claim 36, further comprising means for extracting a selected numberof coded bands from the code string, wherein the band-limiting means isbased on the selected number of coded bands.
 38. The informationdecoding device as claimed in claim 36, wherein the inverse conversionmeans includes means for converting a frequency signal component to atime signal.
 39. The information decoding device as claimed in claim 36,wherein the band-limiting means sets the overlap region between thefirst frequency band and the second frequency band at 0 or a valueproximate to
 0. 40. The information decoding device as claimed in claim36, wherein the filtering frequency overlap region is a quadraturefilter overlap region.
 41. A providing medium for providing processingfor decoding a code string obtained by converting and coding at leastone band of a signal split into a plurality of frequency bands,including a first frequency band adjacent to a second frequency band,the processing comprising: selecting the first frequency band as afrequency band to be decoded and selecting the second frequency band asa frequency band not to be decoded; band-limiting a signal component ofthe first frequency band in a filtering frequency overlap region betweenthe first frequency band and the second frequency band; and inverselyconverting the band-limited signal of the first frequency band.
 42. Aninformation decoding method for decoding a code string obtained bycoding at least one frequency band of a signal split into a plurality ofadjacent frequency bands, including a first frequency band adjacent to asecond frequency band, the method comprising: selecting the firstfrequency band as a frequency band to be decoded and selecting thesecond frequency band as a frequency band not to be decoded; in thefirst frequency band, restoring frequency signal components from thecode string; and inversely converting the restored frequency signalcomponents; wherein the inverse frequency conversion step includeslimiting the value of a signal component existing in a frequency overlapregion between the first frequency band and the second frequency band.43. An information decoding device for decoding a code string obtainedby coding at least one frequency band of a signal split into a pluralityof adjacent frequency bands, including a first frequency and adjacent toa second frequency band, the device comprising: means for selecting thefirst frequency band as a frequency band to be decoded and selecting thesecond frequency band as a frequency band not to be decoded; means forrestoring, in the first frequency band, frequency signal components fromthe code string; and means for inversely converting the restoredfrequency signal components; wherein the inverse frequency conversionmeans includes means for limiting the value of a signal componentexisting in a frequency overlap region between the first frequency bandand the second frequency band.
 44. A providing medium for providingprocessing for decoding a code string obtained by coding at least onefrequency band of a signal split into a plurality of adjacent frequencybands, including a first frequency band adjacent to a second frequencyband, the processing comprising: selecting the first frequency band as afrequency band to be decoded and selecting the second frequency band asa frequency band not to be decoded; in the first frequency band,restoring frequency signal components from the code string; andinversely converting the restored frequency signal components; whereinthe inverse frequency conversion step includes limiting the value of asignal component existing in a frequency overlap region between thefirst frequency band and the second frequency band.
 45. An informationcoding method for coding an input signal, the method comprising:splitting the input signal into a plurality of frequency bands,including a first frequency band adjacent to a second frequency band;identifying the first frequency band to be coded; band-limiting a signalcomponent of the first frequency band in a filtering frequency overlapregion between the first frequency band and the second frequency band.46. The information coding method as claimed in claim 45, wherein thefirst frequency band comprises a band on the low-frequency side of theplurality of frequency bands.
 47. The information coding method asclaimed in claim 45, wherein the signal component is band-limited beforethe input signal is split.
 48. The information coding method as claimedin claim 45, wherein the signal component is band-limited after theinput signal is split.
 49. The information coding method as claimed inclaim 45, wherein the signal component is band-limited before the firstfrequency band is converted.
 50. The information coding method asclaimed in claim 45, wherein the signal component is band-limited afterthe first frequency band is converted.
 51. The information coding methodas claimed in claim 45, wherein a polyphase quadrature filter is used tosplit the input signal.
 52. The information coding method as claimed inclaim 45, wherein converting the first frequency band includesconverting a time signal to a frequency signal component.
 53. Theinformation coding method as claimed in claim 45, wherein converting thefirst frequency band includes converting a time signal split into aplurality of bands to a frequency signal component.
 54. The informationcoding method as claimed in claim 45, wherein the band-limiting thesignal component includes setting the overlap region between the firstfrequency band and the second frequency band at 0 or a value proximateto
 0. 55. The information coding method as claimed in claim 45, whereinthe filtering frequency overlap region is a quadrature filter overlapregion.
 56. An information coding device for coding an input signal, thedevice comprising: means for splitting the input signal into a pluralityof frequency bands, including a first frequency band adjacent to asecond frequency band; means for selecting the first frequency band tobe coded; means for selecting the second frequency band not to be coded;means for band-limiting a signal component of the first frequency bandin a filtering frequency overlap region between the first frequency bandand the second frequency band.
 57. The information coding device asclaimed in claim 56, wherein the first frequency band comprises a bandon the low-frequency side of the plurality of frequency bands.
 58. Theinformation coding device as claimed in claim 56, wherein the signalcomponent is band-limited before the input signal is split.
 59. Theinformation coding device as claimed in claim 56, the signal componentis band-limited after the input signal is split.
 60. The informationcoding device as claimed in claim 56, wherein the signal component isband-limited before the first frequency band is converted.
 61. Theinformation coding device as claimed in claim 56, wherein the signalcomponent is band-limited after the first frequency band is converted.62. The information coding device as claimed in claim 56, wherein apolyphase quadrature filter is used to split the input signal.
 63. Theinformation coding device as claimed in claim 56, wherein the conversionmeans includes means for converting a time signal to a frequency signalcomponent.
 64. The information coding device as claimed in claim 56,wherein the conversion means includes means for converting a time signalsplit into a plurality of bands by the splitting means to a frequencysignal component.
 65. The information coding device as claimed in claim56, wherein the band-limiting means sets the overlap region between thefirst frequency band and the second frequency band at 0 or a valueproximate to
 0. 66. The information coding device as claimed in claim56, wherein the filtering frequency overlap region is a quadraturefilter overlap region.
 67. A providing medium for providing processingfor coding an input signal, the processing comprising: splitting theinput signal into a plurality of frequency bands, including a firstfrequency band adjacent to a second frequency band; selecting the firstfrequency band to be coded; selecting the second frequency band not tobe coded; band-limiting a signal component of the first frequency bandin a filtering frequency overlap region between the first frequency bandand the second frequency band.
 68. An information coding method forcoding an input signal, the method comprising: splitting the inputsignal into a plurality of frequency bands, including a first frequencyband adjacent to a second frequency band; selecting the first frequencyband to be coded; selecting the second frequency band not to be coded;converting the input signal to frequency signal components and codingthe frequency signal components; and generating a code string from thecoded signal components; wherein converting the input signal includesband-limiting a signal component of the first frequency band in afiltering frequency overlap region between the first frequency band andthe second frequency band.
 69. An information coding device for codingan input signal, the device comprising: means for splitting the inputsignal into a plurality of frequency bands, including a first frequencyband adjacent to a second frequency band; selecting the first frequencyband to be coded; selecting the second frequency band not to be coded;means for converting the input signal to frequency signal components andcoding the frequency signal components; and means for generating a codestring from the coded signal components; wherein the converting meansincludes means for band-limiting a signal component of the firstfrequency band in a filtering frequency overlap region between the firstfrequency band and the second frequency band.
 70. A providing medium forproviding processing for coding an input signal, the processingcomprising: splitting the input signal into a plurality of frequencybands, including a first frequency band adjacent to a second frequencyband; selecting the first frequency band to be coded; selecting thesecond frequency band not to be coded; converting the input signal tofrequency signal components and coding the frequency signal components;and generating a code string from the coded signal components; whereinconverting the time signal includes band-limiting a signal component ofthe first frequency band in a filtering frequency overlap region betweenthe first frequency band and the second frequency band.
 71. A providingmedium for providing a signal coded by an information coding method forcoding an input signal, the information coding method comprising:splitting the input signal into a plurality of frequency bands,including a first frequency band adjacent to a second frequency band;selecting the first frequency band to be coded; selecting the secondfrequency band not to be coded; band-limiting a signal component of thefirst frequency band in a filtering frequency overlap region between thefirst frequency band and the second frequency band; and converting theband-limited signal.
 72. A providing medium for providing a signal codedby an information coding method for coding an input signal, theinformation coding method comprising; splitting the input signal into aplurality of frequency bands, including a first frequency band adjacentto a second frequency band; selecting the first frequency band to becoded; selecting the second frequency band not to be coded; convertingthe input signal to frequency signal components and coding the frequencysignal components; and generating a code string from the coded signalcomponents; wherein converting the time signal includes band-limiting asignal component of the first frequency band in a filtering frequencyoverlap region between the first frequency band and the second frequencyband.